Ageing (British and Australian English) or aging (American and Canadian English) is the accumulation of changes in an organism or object over time. Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Research shows that even late in life potential exists for physical, mental, and social growth and development. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years — and months for young children. A person's birthday is often an important event. Roughly 100,000 people worldwide die each day of age-related causes.

The term "ageing" is somewhat ambiguous. Distinctions may be made between "universal ageing" (age changes that all people share) and "probabilistic ageing" (age changes that may happen to some, but not all people as they grow older, such as the onset of type two diabetes). Chronological ageing, referring to how old a person is, is arguably the most straightforward definition of ageing and may be distinguished from "social ageing" (society's expectations of how people should act as they grow older) and "biological ageing" (an organism's physical state as it ages). There is also a distinction between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis).

Differences are sometimes made between populations of elderly people. Divisions are sometimes made between the young old (65–74), the middle old (75–84) and the oldest old (85+). However, problematic in this is that chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities. Each nation, government and non-government organization has different ways of classifying age.

Population ageing is the increase in the number and proportion of older people in society. Population ageing has three possible causes: migration, longer life expectancy (decreased death rate), and decreased birth rate. Ageing has a significant impact on society. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting. Thus, the aged have comparatively more political influence.

Alternative spellings

• ageing (Commonwealth English)

aging

1. Present participle of age.

Quotes reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895).

• An aged Christian with the snow of time on his head may remind us that those points of earth are whitest that are nearest heaven.
  • Edwin Hubbell Chapin, p. 439.

• Thanks to that regular and temperate course of life I have ever lived, I am still capable of taking an active part in these public scenes of business. In fine, he who fills up every hour of his life in such kind of labors as those I have mentioned, will insensibly slide into old age without perceiving its arrival; and his powers, instead of being suddenly and prematurely extinguished, will gradually decline by the gentle and natural effect of accumulated years.
  • Cicero, p. 438.

• The day of life spent in honest and benevolent labor comes in hope to an evening calm and lovely; and though the sun declines, the shadows that he leaves behind are only to curtain the spirit unto rest.
  • Henry Giles, p. 438.

• It is not so bad a thing to grow old; it is only getting a little nearer home; a little nearer to immortal youth.
  • Arthur Henry Kenney, p. 439.

• Age is not all decay; it is the ripening, the swelling of the fresh life within, that withers and bursts the husk.
  • George MacDonald, p. 439.

• The second childhood of a saint is the early infancy of a happy immortality, as we believe.
  • William Mountford, p. 438.

• The years of old age are stalls in the cathedral of life in which for aged men to sit and listen and meditate and be patient till the service is over, and in which they may get themselves ready to say "Amen" at the last, with all their hearts and souls and strength.
  • William Mountford, p. 439.

• My God! my time is in Thine hands. Should it please Thee to lengthen my life, and complete, as Thou hast begun, the work of blanching my locks, grant me grace to wear them as a crown of unsullied honor.
  • Christian Scriver, p. 439.